1. Field of the Invention
The invention relates to a magnetic storage device which stores information in magneto-resistive elements.
2. Description of the Related Art
MRAM (Magnetic Random Access Memory) has been receiving attention recently as a storage device for use in information processors such as computers and communication apparatuses. MRAM stores data by means of magnetism. Since the direction of magnetization can be maintained without use of any electrical means, it is possible to avoid any loss of information upon power-down as happens with DRAM (Dynamic Random Access Memory) and SRAM (Static RAM), or other volatile memories. When compared with conventional nonvolatile storing means such as flash EEPROM and hard disk units, MRAM is superior in terms of access speed, reliability, power consumption, and the like. It can thus be said that MRAM is capable of replacing all the functions of volatile memories such as DRAM and SRAM and those of nonvolatile storing means such as flash EEPROM and hard disk units (see Japanese Patent Publication No. 3466470).
Consider, for example, the case of developing information equipment that targets so-called ubiquitous computing in which information processing is available regardless of location. A requirement of such ubiquitous computing includes a storage device that is capable of high speed processing with reduced power consumption and that can also avoid loss of information, even upon power-down. MRAM has the potential to meet both of these requirements, and is expected to be adopted by many information apparatuses in the future.
In particular, for tablets, portable information terminals, and the like that are intended to be carried about daily, it is often difficult to secure sufficient power supply. Thus, in order to perform a great deal of information processing in severe use environments, even MRAM of low power consumption requires a further reduction in power consumption during information processing.
Among examples of technologies intended to advance the power savings of MRAM are magnetic storage devices described in Japanese Patent Application Laid-Open Nos. 2000-90658 and 2004-128430. These magnetic storage devices include, in each of their storage areas (memory cells), a bit line, a word line arranged orthogonal to the bit line, a tunneling magneto-resistive (TMR) element arranged at the intersection between the bit line and the word line, and the like. These magnetic storage devices further include a yoke (field control layer) which is arranged near each TMR element so as to surround the bit line and the word line of the same. The yoke is made of a ferromagnetic body having high permeability, and functions to reduce the occurrence of flux leakage from the bit line or the word line, thereby concentrating the flux into the TMR element. This makes it possible to produce a field necessary for inverting the magnetization of the TMR element, even at low power consumption.
It should be appreciated that a TMR element includes: a first magnetic layer (magnetic sensing layer) whose direction of magnetization changes with an external field; a second magnetic layer having a fixed direction of magnetization; and a nonmagnetic insulating layer interposed between the first magnetic layer and the second magnetic layer. This TMR element stores binary data by controlling the direction of magnetization of the first magnetic layer so that the direction of magnetization is parallel or antiparallel with the direction of magnetization of the second magnetic layer.
The smaller the size of the TMR elements used in magnetic storage devices, the greater the effect of the diamagnetic fields occurring inside, and thus the stronger the write fields must be made. Consequently, a contradiction arises because the write currents must be increased if greater integration of the magnetic storage devices is intended. It is generally noted that magnetic storage devices are difficult to miniaturize and difficult to enhance the recoding density thereof.